1) Field of the Invention
This invention relates to a process for producing an economic protein by using silkworms, and more specifically to a process for producing an economic protein by efficiently infecting silkworms p.o. with a virus in which a gene coding the target protein has been inserted (hereafter called the "recombinant virus").
2) Description of the Related Art
In recent years, active research has been conducted with respect to the technology for producing an economic protein with silkworms by making use of recombinant DNA technology. For example, processes using silkworm nuclear polyhedrosis virus as a vector have been reported (Japanese Patent Application Laid-Open Nos. 9288/1986 and 208276/1987, etc.).
According to these processes, a recombinant virus is created by replacing the polyhedral gene of silkworm nuclear polyhedrosis virus with another gene which codes an economic substance. Silkworms of fifth instar are inoculated and infected with the recombinant virus. Four to eight days later, the economic substance which has been produced by the recombinant virus in silkworm cells and secreted in the body fluid in the course of growth of the recombinant virus is collected, isolated and then purified.
Such recombinant DNA technology, which uses silkworms, is considered preferable from the standpoints of the activity, antigenicity and the like of the economic substances to be produced, because the silkworm as a host is closer genetically to man than Escherichia coli and yeast which have been used conventionally.
Conventional silkworm-dependent recombinant DNA methods are, however, accompanied by serious problems.
As a method for infecting and inoculating silkworms with a recombinant virus, two methods have been studied, one being the cutaneous infection method in which silkworms are injected one by one with a suspension of the virus to infect them with the virus, and the other the oral infection method in which a suspension of the virus is added to the silkworms' feed to infect them via the digestive tract.
Of these methods, the oral infection method is extremely efficient for the mass production of an economic substance as infection can be completed, for example, by coating artificial feed with a virus suspension and administering it in one episode. This is however not the case, for example, when silkworm nuclear polyhedrosis virus is employed. The recombinant virus produces a target protein in place of a polyhedral protein which is supposed to envelope the virus itself and hence to protect it from dry air and ultraviolet rays in the external atmosphere and, when administered to the silkworm, from the digestive juice of the silkworm. The recombinant virus taken into the digestive tract of the silkworm by the oral infection method is readily inactivated by the digestive juice of the silkworm, so that the silkworm is not infected by it at all.
For the reasons mentioned above, it is the common practice that infection by a recombinant virus is conducted by cutaneous infection. According to cutaneous infection, silkworms must be injected one by one immediately after their ecdysis and, moreover, the injection requires skill and time as it must be performed without damages to various organs in the body of silkworm. Cutaneous infection, therefore, can by no means be adopted for the industrial production of an economic substance.